Separator plates are well known coponents of batteries and other electrochemical devices. In these devices they are used to separate adjacent cells. In fuel cells, for example, they serve the function of preventing the mixing of a fuel gas, such as hydrogen, disposed on one side of the plate, with an oxidant, such as air, disposed on the other side thereof. They must, therefore, be highly impermeable to a gas such as hydrogen and highly electrically conductive. It has been particularly difficult developing separator plates for use in phosphoric acid electrolyte due to the highly corrosive nature of the acid, particularly at high temperatures. Only a few years ago fuel cells operated at temperatures between 275.degree. F. (135.degree. C.) and 325.degree. F. (163.degree. C.). Today there is a need for phosphoric acid electrolyte fuel cell separator plates to be corrosion resistant to the electrolyte for long periods of time (years) at operating temperatures as high as 425.degree. F. (218.degree. C.); and they must be strong, particularly in terms of flexural strength, which is an indication of the plates' ability to withstand high pressure loads, differential thermal expansion of mating components, and numerous thermal cycles without cracking or breaking. It has also been desirable to make these plates thinner for improved electrical and thermal conductivity and for more economical and more versatile fuel cell configurations. This makes them even more difficult to fabricate with the requisite strength and impermeability.
Graphite is one of the few known relatively inexpensive substances highly resistant to corrosion in hot phosphoric acid. There is considerable prior art relating to dense graphite articles made by molding and then heat treating mixtures of graphite or carbon powder and a carbonizable resin. Representative of this art are the following U.S. Pat. Nos. 3,283,040; 3,708,451; 3,838,188; 3,907,950; 3,969,124; 3,624,569; and 3,716,069. The last two of the above-listed patents are owned by the same assignee as the present application and are specifically directed to molding separator plates and the like for use in phosphoric acid fuel cells. Although there are some common threads running through these above-mentioned references, the distinctions are even more notable. For example, in U.S. Pat. No. 3,708,451 a quantity of camphor is mixed with the graphite and resin prior to molding and is considered critical to obtaining a graphite product having a "virtually impermeable surface". A resin content of from 30-60 weight percent is indicated, with examples of possible resins being polymerized furfuryl alcohol, pitch and furans, none of which are believed to be totally acceptable for use in phosphoric acid cells. The patent teaches that the graphite might be in the form of a powder with all the particles being less than 5 microns (for a smooth surface) or having a range up to 500 microns; or graphite fibers may also be used.
In U.S. Pat. No. 3,283,040 a mixture of nongraphitic carbon (i.e., lampblack or carbon black) and coal tar pitch are molded to form a carbon body which is graphitized by heating. Densities of up to 1.71 gm/cc are achieved.
U.S. Pat. No. 3,907,950 is concerned with making "spark erosion electrodes". The electrodes are molded from a blend of not more than 14 percent carbonizable resin (such as a novolac resin) and graphite powder having a particle size of less than 200 mesh (174 microns). Densities of up to 1.70 gm/cc were attained. There is nothing in this patent which would appear to be relevant to a person having skill in the fuel cell art as regards the composition and fabrication of a fuel cell separator plate. The same is true for U.S. Pat. No. 3,838,188 which is also concerned with molding carbonaceous electrical discharge machining electrodes.
U.S. Pat. No. 3,969,124 describes molding and subsequently graphitizing a mixture of non-graphitic carbon and graphite particles and a phenolic resin to form electrodes, anodes and crucibles. The patent teaches 20-50 percent resin, with 20-25 percent being preferred. The patent does recognize that the graphite particle size distribution can and does have an effect on the properties of the finished article. It indicates that 50% of the particles must be less than 10 microns in diameter. Chemical vapor deposition is recommended for increasing the density, and it is recommended that 10-30 percent graphite fibers or whiskers be added to the molding mixture for increased strength.
U.S. Pat. No. 3,634,569 is directed to molding thin graphite plates useful as separator plates in phosphoric acid fuel cells. The recommended molding mixture is 5-25 percent thermosetting phenolic resin binder and 75-90 percent powdered graphite. A recommended graphite particle size distribution is set fourth in Table I and calls for a maximum of 12 percent of the particles being below 50 microns. A plate made by the process is described in Example I of that patent; and some of its properties are listed in column 1 of Table II of that patent. Note that this plate is not graphitized since the maximum heat treat treatment temperature was about 400.degree. F. (205.degree. C.).
U.S. Pat. No. 3,716,609 describes a process for forming fuel cell separator plates from a molding mixture comprising 60-90 percent graphite powder and 10-40 percent polyphenylene sulfide (PPS) resin particles. A preferred composition is 85 percent graphite powder and 15 percent resin powder. Particle size distributions for both the resin and the graphite are taught. Note that the maximum permitted amount of particles in the smaller than 45 micron size range was about 20 percent. This was the best plate known prior to the present invention. However it was designed for long term operation in phosphoric acid at temperatures not exceeding about 325.degree. F. The plate is not and cannot be subject to graphitization temperatures since above 600.degree. F. PPS loses all its strength and shape holding characteristics. Some properties and characteristics of parts made by the described process are set forth in Table II of that patent.
Despite this plethora of art in the field of dense carbon articles and fuel cell separator plates, there is no teaching of a thin plate which can withstand use over an extended period of time in the environment of phosphoric acid fuel cell operating at temperatures greater than about 325.degree. F.